Nitric oxide (NO) has historically been viewed as an environmental pollutant and as an atmospheric health hazard, but recently it has been shown to play an active role in biological signaling events in the cardiovascular, immune, and nervous systems. Regulation of NO production is of immense importance because low cellular concentrations of NO are beneficial for vasodilation and immune activity such as preventing tumor growth. Overproduction of cellular NO, however, can lead to the proliferation of reactive NO species which have been implicated in carcinogenesis and several degenerative neurological disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. Although several NO sensors currently exist, they all have limitations which diminish the facility of in vivo use. While NO detection has been accomplished in vivo, the reversible sensing of NO remains an important factor in quantifying NO concentrations in different cellular locales and in response to external stimuli. This proposal involves the design of a reversible binding site for NO coupled with photoinduced electron transfer (PET) to allow for fluorescent imaging. The ultimate goal of this research is to develop a reversible fluorescent probe for cellular levels of NO which can be used in vivo for the temporal and spatial detection of NO. The proposed family of fluorescent sensors use paramagnetic Fe(lll) (S = 1/2) centers serving a dual role as both fluorescence quencher and binding site for NO. Coordination of NO to the Fe(lll) center will afford a diamagnetic {Fe-NO}6 complex, allowing for the restoration of fluorescence on the pendant fluorophore. The ligand scaffold provides a unique platform where both the ligand electronics and the hydrophobic pocket for NO binding can be finely tuned. Various ligand substitutions and fluorophores will be investigated to tune the NO binding properties of the Fe(lll) center to favor reversible NO binding;functional group incorporation into the ligand can allow for cellular targeting. PUBLIC HEALTH RELEVANCE Overproduction of cellular nitric oxide (NO) can lead to the proliferation of reactive NO species (RNOS) which have been implicated in carcinogenesis and several neurological disorders, including Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis. A detailed understanding of the role of NO as a chemical messenger in biological systems has implications that span across many fields of medicine. In this application, we propose the design of fluorescent probes for NO which will allow for the detection and quantification of NO in cellular components.